Range cameras, also referred to as range imaging cameras, range imaging systems, and ranging devices, determine both distance and intensity for objects in a scene to be imaged. Range cameras are used in a variety of applications to determine the distance between an object and a camera. While typical range cameras are able to accurately determine distances to stationary objects, they are unable to accurately determine the distances to moving objects because they operate on an assumption that signal data received from a particular location in a scene is reflected to the camera from the same object located at the same distance from the camera during each of several samplings of the scene-dependent reflected light. When the objects move during imaging, this assumption may fail and errors may occur.
The difficulties in determining distance to moving objects are the result of the fundamental operation of typical range cameras. Range cameras typically utilize Amplitude Modulated Continuous Wave (“AMCW”) technology to determine the distance between the camera and an object. Cameras using AMCW technology use a light source to emit an intensity modulated illumination signal to illuminate a scene to be imaged. The signal is then reflected back to the range camera by objects within the scene. Because the reflected signal slips out of phase with the emitted signal as the signal travels away from and back to the camera, multiple images of the reflected light are captured by a specialized sensor chip to record the phase differences at set periods of time. A processor is then used to compare the multiple images to each other and to the emitted intensity modulated illumination signal to determine phase changes in the intensity modulation of the light. The phase changes are then analyzed to determine the distance between the objects and the range camera, as well as to determine the size, shape, and location of the objects.
FIG. 1 illustrates a simplified version of a typical range camera. As shown in FIG. 1, a typical AMCW range camera 100 may include a signalling device 101 to emit an intensity modulated illumination signal towards a scene to be imaged and an image sensor 102 to capture images using the portions of the emitted signal that are reflected back to the system 100 from the objects within the scene. A processor 103 then compares the emitted signal with captured images to determine the intensity and the phase shift for each pixel in the captured image. By doing so, the processor is able to determine the distance the objects in the scene are from the system 100. The system 100 may optionally contain an input device 104 to control the system 100, a display 105, and a memory 106; or these devices 104, 105, and 106, may be in the form of a separate computer system connected by an interface cable. These devices may be chosen from among any of the known devices for performing such functions.
There are many known ways of modulating the signalling device 101 and the image sensor 102. For example, the signalling device 101 may simply be a light source that is cycled on and off very quickly, while the image sensor 102 may be modulated using a high speed shutter or by controlling the gain or sensitivity of the image sensor 102. The processor 103 may also include a signal generator, or a similar device, to assist in modulating the signalling device 101 and the image sensor 102. However, a signal generator may also be separately provided.
FIG. 2 is a functional diagram of the standard range camera described above with reference to FIG. 1. As shown in FIG. 2, the intensity modulation envelope of the illumination signal emitted from the signaling device 101 is shown as a sinusoidal wave shape (although it may also be modulated by a square wave). The signal is then reflected from various objects 21, 22, and 23 within the scene being imaged. As shown in FIG. 2, the light reflected back to the image sensor 102 will travel different distances based on the relative distances of the objects 21, 22, and 23 reflecting the light. This will result in the received light's modulation having different phases based on the distance traveled. Although not shown in FIG. 2, the amplitude of the waveform may also change based on the distance traveled and surface scattering properties of the objects.